Chromatic dispersion is based on the principal that different colored pulses of light, with different wavelengths, travel at different speeds, even within the same mode, and is the sum of material dispersion and waveguide dispersion. Material dispersion is caused by the variation in the refractive index of the glass of a fiber as a function of the optical frequency. Waveguide dispersion is caused by the distribution of light between the core of a fiber and the cladding of a fiber, especially with regard to a single-mode fiber. Chromatic dispersion concerns are compounded in today's high-speed transmission optical networks.
Currently, the chromatic dispersion penalty, or chromatic dispersion tolerance, of optical transmitters is measured or tested using physical lengths of conventional non-dispersion shifted fiber (NDSF) conforming to International Telecommunications Union (ITU) standard G.652. In general, NDSF has its dispersion null point, at which waveguide and material chromatic dispersion cancel each other out, near a wavelength of 1310 nm. The specification for the amount of dispersion that an optical transmitter must tolerate is given as a uniform value based on the bit rate of the optical transmitter, independent of the optical transmitter's wavelength. For example, 2.5 Gb/s optical signals generally can tolerate up to 16000 ps/nm of dispersion, 10 Gb/s optical signals generally can tolerate up to 1000 ps/nm of dispersion, and 40 Gb/s optical signals generally can tolerate up to 60 ps/nm of dispersion, However, the amount of dispersion present in NDSF per unit length is strongly dependent upon the optical transmitter's wavelength, and varies approximately linearly from 16.2 ps/nm/km at 1530 nm to 17.5 ps/nm/km at 1550 nm to 18.5 ps/nm/km at 1565 nm. Thus, the wavelength dependent dispersion slope of NDSF is approximately 0.061 ps/nm2/km.
The dispersion penalty, or dispersion tolerance, of optical transmitters is measured or tested in order to determine whether or not a given optical transmitter meets the required specification. As described above, existing measurement or testing methods use using physical lengths of conventional NDSF with known dispersion characteristics. Such methods, however, do not account for the wavelength-dependent dispersion slope of NDSF.
For the ease of measurement or testing during manufacturing, it is desirable for a component vendor to use only a single setup with a single physical length of NDSF for optical transmitters of all wavelengths. In order to guarantee a wavelength independent dispersion tolerance, a physical length of fiber that provides the specified amount of dispersion is computed at the short end of the applicable wavelength spectrum. In other words, when using a single physical length of NDSF to perform a measurement or test of the dispersion penalty of an optical transmitter at any given wavelength to a wavelength independent amount of total dispersion, the physical length of NDSF needed is determined at the short end of the applicable wavelength spectrum. As a result, due to the wavelength dependent dispersion slope of NDSF, the dispersion tolerance at all longer wavelengths is increasingly greater than the specification, resulting in the over testing of the components, lower yield, and increased component cost. For example, if the wavelength independent dispersion specification of a 10 Gb/s optical transmitter is 1600 ps/nm, then approximately 100 km of NDSF is needed to generate 1600 ps/nm dispersion at 1530 nm. At 1565 nm, the dispersion is 1840 ps/nm, or 114% of the wavelength independent dispersion specification. As the dispersion penalty as a function of dispersion is effectively quadratic, this difference in total dispersion amount results in an over testing of the dispersion penalty by 132%.
Thus, what is needed in the art is a method for the wavelength independent measurement or testing of the dispersion penalty, or dispersion tolerance, of optical transmitters that comprises a single setup, measures or tests the components only to a specified amount of dispersion, and not beyond, improves yield, and reduces component cost.